Martensitic stainless steel for oil wells and method for manufacturing martensitic stainless steel pipe for oil wells

ABSTRACT

A martensitic stainless steel for oil wells, which comprises specified quantities of C, Si, Mn, P, S, Cr, Al, Ni, Cu, Mo, Ti and N, and the balance being Fe and impurities, has not only a satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide but also an excellent low temperature toughness. Therefore, it can be used in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas. This steel pipe stands good for a use in the above-mentioned severe environment can be easily manufactured by the present invention method.

The disclosure of Japanese Patent Application No. 2005-136329 field in Japan on May 9, 2005, including specifications and claims, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a martensitic stainless steel for oil wells and a method for manufacturing a martensitic stainless steel pipe for oil wells; more specifically, a martensitic stainless steel for oil wells suitable for use in cold areas in a severe corrosive environment, containing corrosive materials such as hydrogen sulfide (H₂S), carbon dioxide (CO₂) and chloride ion (Cl⁻); and a method for manufacturing a martensitic stainless steel pipe using such a steel. Further specifically, the present invention relates to a martensitic stainless steel suitable for use in cold areas for production facilities of petroleum and natural gas, for carbon dioxide removal facilities, for geothermal power generation facilities, and for oil country tubular goods, such as seamless steel pipes, electric resistance welded steel pipes, and laser welded steel pipes used in oil wells and natural gas wells, and a method for manufacturing a martensitic stainless steel pipe using such a steel.

In recent years, the environment of wells for collecting petroleum or natural gas has become increasingly more severe, and oil wells and gas wells containing corrosive gas, such as hydrogen sulfide and carbon dioxide, have been actively developed.

Therefore, in oil country tubular goods which are used for extraction of petroleum or natural gas in a severe environment as mentioned above, the deterioration of the raw materials by corrosion is a major problem.

That is to say, steel pipes which use carbon steel or low alloy steel as raw materials have generally been used in steel pipes for oil wells or gas wells which contain no carbon dioxide, while those using carbon steel or low alloy steel cannot ensure sufficient corrosion resistance in oil wells or gas wells containing large quantities carbon dioxide, and steel pipes using steels increased in contents of alloy elements as raw materials are thus used therefor.

Concretely, for raw materials of the steel pipes for oil wells containing carbon dioxide in large quantities, 13Cr-type martensitic stainless steel as represented as SUS 420J1 in JIS is frequently used.

However, the said 13Cr-type martensitic stainless steel as represented as SUS 420J1 is poor in corrosion resistance to hydrogen sulfide, and apt to cause sulfide stress cracking (hereinafter referred to as “SSC”) in an environment which contains both carbon dioxide and hydrogen sulfide. Therefore, in an actual situation its use should be limited.

On the other hand, oil wells and gas wells simultaneously containing chloride ion, wet carbon dioxide and trace hydrogen sulfide have been actively developed even in cold areas, and thus, a steel which has not only satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide but also excellent toughness in low temperature, has been increasingly demanded.

Therefore, a steel for oil country tubular goods containing, by mass percent, 12 to 13.5% of Cr is disclosed in Patent Document 1.

Patent Document 1: Japanese Laid-Open Patent Publication No. 60-52525

DISCLOSURE OF THE INVENTION SUBJECT TO BE SOLVED BY THE INVENTION

The main objectives of the present invention are to provide a martensitic stainless steel for oil wells with high toughness and high SSC resistance, which is excellent in low temperature toughness as well as satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide, and is sufficiently usable even in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas, and also to provide a method for manufacturing a martensitic stainless steel pipe for oil wells using the aforementioned steel.

The steel for oil country tubular goods disclosed in Patent Document 1 cannot necessarily ensure excellent low temperature toughness.

That is to say, in the steel for oil country tubular goods disclosed in Patent Document 1, the content of Ni is limited to not more than 0.10% by mass percent in order to enhance the SSC resistance or pitting resistance. Accordingly, this steel for oil country tubular goods could not ensure sufficient low temperature toughness in spite of satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide.

Therefore, with respect to various martensitic stainless steels containing, by mass percent, not less than 12.5% of Cr in order to ensure carbon dioxide corrosion resistance, and changed in strength to 558 to 655 MPa in terms of yield strength (YS), the present inventors variously examined and studied for SSC susceptibility in a coexistent environment of chloride ion, wet carbon dioxide and trace hydrogen sulfide, and further examined and studied for low temperature toughness. As a result, the following findings (a) to (c) were obtained.

(a) In the above-mentioned martensitic stainless steel containing, by mass percent, not less than 12.5% of Cr, satisfactory SSC resistance could be obtained even with a Ni content exceeding 0.10% by mass percent.

(b) In the above-mentioned martensitic stainless steel, satisfactory low temperature toughness could be obtained even with the Ni content of not more than 0.10% by mass percent.

(c) Satisfactory low temperature toughness of the above-mentioned martensitic stainless steel cannot be necessarily obtained only by increasing a Ni content more than 0.10% by mass percent.

Therefore, with respect to the influence of the chemical composition and microstructure after tempering, particularly the precipitates in the microstructure of above-mentioned steel on the SSC resistance and low temperature toughness in the above-mentioned environment, further detailed examinations were carried out. As a result, the following findings (d) to (g) were obtained

(d) When the content of Al of the above-mentioned martensitic stainless steel is not more than 0.010% by mass percent, the main precipitates present in the tempered microstructure are M₂₃C₆ and MC (“M” means a metal element).

(e) When the contents of Al and Ni of the above-mentioned martensitic stainless steel are not more than 0.010%, and more than 0.10% and not more than 0.2% by mass percent, respectively, satisfactory SSC resistance can be obtained in the above-mentioned environment, with extremely satisfactory low temperature toughness.

(f) When the contents of Al and Ni of the above-mentioned martensitic stainless steel are not more than 0.010%, and not more than 0.10% by mass percent, respectively, satisfactory SSC resistance can be obtained in the above-mentioned environment. The low temperature toughness is also satisfactory in spite of a low content of Ni.

(g) When the content of Al of the above-mentioned martensitic stainless steel exceeds 0.010% by mass percent, the main precipitates present in the tempered microstructure are M₂₃C₆ and AlN. In this case, even if the content of Ni is more than 0.10% and not more than 0.2%, the low temperature toughness is poor.

The present invention has been accomplished on the basis of the above findings.

MEANS FOR SOLVING THE PROBLEM

The gists of the present invention are martensitic stainless steels for oil wells, described in the following (1) to (4), and a method for manufacturing a martensitic stainless steel pipe for oil wells, described in the following (5).

(1) A martensitic stainless steel for oil wells, which comprises by mass percent, C: 0.16 to 0.22%, Si: 0.1 to 0.8%, Mn: 0.25 to 1.00%, P: not more than 0.025%, S: not more than 0.010%, Cr: 12.0 to 13.5%, Al: not more than 0.010%, Ni: 0 to 0.2%, Cu: 0 to 0.10%, Mo: 0 to 0.20%, Ti: 0 to 0.050% and N: 0.01 to 0.1%, with the balance being Fe and impurities.

(2) The martensitic stainless steel for oil wells according to the above-mentioned (1), which further contains either one or both elements selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2% in lieu of part of Fe.

(3) The martensitic stainless steel for oil wells according to the above-mentioned (1), which further contains one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005% in lieu of part of Fe.

(4) The martensitic stainless steel for oil wells according to the above-mentioned (1), which further contains either one or both elements selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2%, and one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005% in lieu of part of Fe.

(5) A method for manufacturing a martensitic stainless steel pipe for oil wells, which comprises:

heating the martensitic stainless steel pipe having the chemical composition according to any one of above (1) to (4), to a temperature range of 920 to 1050° C. followed by air cooling; and then,

performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point.

The above-mentioned “air cooling” includes not only so-called “forced cooling” but also “standing to cool” in the atmospheric air.

The inventions for the “martensitic stainless steels” described in the aforementioned (1) to (4), and the invention for the “method for manufacturing a martensitic stainless steel pipe for oil wells” described in above (5) are called “Invention (1)” to “Invention (5)”, respectively, or often collectively called “the present invention”.

EFFECT OF THE INVENTION

The martensitic stainless steel for oil wells, according to the present invention can be used in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas since it is excellent in low temperature toughness as well as satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide. Further according to the method for manufacturing a martensitic stainless steel pipe for oil wells of the present invention, a steel pipe which stands good for a use in the above-mentioned severe environment can be easily manufactured.

BEST MODE FOR CARRYING OUT THE INVENTION

Requirements of the present invention will next be described in detail. In the following description, the symbol “%” indicative of the content of each element means “mass percent”.

(A) Chemical Compositions:

C: 0.16 to 0.22%

C is an element necessary for giving a desired strength to steel. C has also the effect of easily making the microstructure of steel by air cooling the martensite in order to enhance the SSC resistance. The microstructure of steel is desirably made into a martensitic microstructure (desirably, not less than 95%) in order to improve the SSC resistance in high strength steel stocks. Since C is an austenite-forming element, the ratio of martensite can be easily increased by increasing the content of C. However, when the content of C is less than 0.16%, it is difficult to obtain each of the above-mentioned effects. On the other hand, a content more than 0.22% causes a remarkable deterioration in toughness. Therefore, the content of C is set to 0.16 to 0.22%.

Si: 0.1 to 0.8%

Si has a deoxidizing effect. However, when the content of Si is less than 0.1%, the effect of adding of Si is insufficient. On the other hand, a content more than 0.8% causes a deterioration in hot workability as well as a deterioration in toughness. Therefore, the content of Si is set to 0.1 to 0.8%. The upper limit thereof is preferably set to 0.4%.

Mn: 0.25 to 1.00%

Mn has effects of extending the austenite range and also improving strength or toughness. However, when the content of Mn is less than 0.25%, the above effects cannot be obtained to a desired degree. On the other hand, when the content of Mn exceeds 1.00%, carbon dioxide corrosion susceptibility is remarkably increased which reduces the carbon dioxide corrosion resistance. Therefore, the content of Mn is set to 0.25 to 1.00%.

P: not more than 0.025%

P is an impurity of steel, which causes deterioration in both toughness and SSC resistance. A content more than 0.025%, particularly, makes a marked deterioration in both toughness and SSC resistance. Therefore, the content of P is set to not more than 0.025%. The content of P is desirably as small as possible.

S: not more than 0.010%

S is an impurity of steel which causes deterioration in both toughness and SSC resistance. A content more than 0.010%, particularly, makes a marked deterioration in both toughness and SSC resistance. Therefore, the content of S is set to 0.010%. The content of S is desirably as small as possible.

Cr: 12.0 to 13.5%

Cr has the effect of enhancing carbon dioxide corrosion resistance. When the content of Cr is less than 12.0%, sufficient carbon dioxide corrosion resistance cannot be ensured. On the other hand, since the generation rate of δ-ferrite is increased when the content of Cr exceeds 13.5%, a microstructure with a high ratio of martensite preferable for SSC resistance, particularly, a martensitic microstructure of not less than 95% cannot be obtained by air cooling. Therefore, the content of Cr is set to 12.0 to 13.5%. The lower limit of the Cr content is more preferably set to 12.3%, and the upper limit more preferably to 13.2%.

Al: not more than 0.010%

When the content of Al is increased, the main precipitates present in the tempered microstructure are M₂₃C₆ and AlN, and they cause a deterioration in SSC resistance. The Al content more than 0.010%, particularly, makes a marked deterioration in SSC resistance. In this case, the low temperature toughness is poor even if the content of Ni is more than 0.10% and not more than 0.2%. On the other hand, when the content of Al is not more than 0.010%, the main precipitates present in the tempered microstructure are M₂₃C₆ and MC. Further, when the content of Al is not more than 0.010% and the content of Ni is more than 0.10% and not more than 0.2%, satisfactory SSC resistance and extremely satisfactory low temperature toughness can be obtained. When the content of Al is not more than 0.010%, satisfactory low temperature toughness can be ensured even if the content of Ni is not more than 0.10%.

Accordingly, the content of Al is preferably controlled to an impurity level, and set to not more than 0.010%. The content of Al is set preferably to not more than 0.005%, more preferably, not more than 0.003%.

Ni: 0 to 0.2%

Ni may be optionally added. When added, it has the effect of improving the strength and the low temperature toughness. However, an increased Ni content causes an increase in SSC susceptibility and, particularly, when the content of Ni exceeds 0.2%, the SSC susceptibility is markedly increased, resulting in a deterioration in SSC resistance. Therefore, the content of Ni is set to 0 to 0.2%. In order to ensure the above effect, Ni is preferably contained in an amount more than 0.10%.

Cu: 0 to 0.10%

Cu may be optionally added. When added, it has the effect of improving the uniform corrosion resistance in a high temperature range. However, when the content of Cu is increased, particularly to more than 0.10%, susceptibility to local corrosion (that is to say, pitting susceptibility) is enhanced. Therefore, the content of Cu is set to 0 to 0.10%.

Mo: 0 to 0.20%

Mo may be optionally added. When added, it has the effect of improving the uniform corrosion resistance in a high temperature range. However, when the content of Mo is increased, particularly to more than 0.20%, the generation of δ-ferrite is enhanced by heating prior to air cooling, resulting in increased susceptibility to local corrosion (that is to say, pitting susceptibility) as well as a deterioration in strength. Therefore, the content of Mo is set to 0 to 0.20%.

Ti: 0 to 0.050%

Ti may be optionally added. When added, it has the effect of fixing the N in the steel as TiN, and so the free N is reduced. However, when the content of Ti is increased, particularly to more than 0.050%, the toughness is deteriorated. Therefore, the content of Ti is set to 0 to 0.050%.

N: 0.01 to 0.1%

N is an austenite-forming element. Therefore, the ratio of martensite can be easily increased by increasing the content of N, whereby the SSC resistance can be effectively enhanced. However, when the content of N is less than 0.01%, it is difficult to obtain the said effect. On the other hand, when the content of N is excessively high, the carbon dioxide corrosion resistance is deteriorated due to the formation of a large quantity of nitrides of Cr. Particularly, a N content more than 0.1% makes a marked deterioration in carbon dioxide corrosion resistance. Therefore, the content of N is set to 0.01 to 0.1%. The upper limit of the N content is set more preferably to 0.05%, and further preferably to 0.04%.

The martensitic stainless steel for oil wells according to the said Invention (1) is regulated, for the above-mentioned reasons, to contain C to N in the above-mentioned ranges with the balance being Fe and impurities.

The martensitic stainless steel for oil wells according to the present invention may include in lieu of part of Fe, one or more elements selected from at least one of the first group and the second group described later as optionally additive elements as the occasion demands.

The optionally additive elements will now be described.

First group: Nb: 0.020 to 0.045% and V: 0.01 to 0.2%

Nb is an element effective for enhancing the strength of steel. However, when the content of Nb is less than 0.020%, the said effect is insufficient. On the other hand, when the content of Nb exceeds 0.045%, the said effect is either saturated, or a deterioration in strength is reversely caused because of formation of δ-ferrite. Therefore, the content of Nb to be added is set to 0.020 to 0.045%.

V is an element effective for enhancing the strength of steel. However, when the content of V is less than 0.01%, the said effect is insufficient. On the other hand, when the content of V exceeds 0.2%, the said effect is either saturated, or a deterioration in strength is reversely caused because of formation of δ-ferrite. Therefore, the content of V to be added is set to 0.01 to 0.2%.

The above-mentioned Nb and V can be added singly or in combination.

Second group: Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005%

Ca is an element effective for enhancing the hot workability of steel. However, when the content of Ca is less than 0.0002%, the said effect cannot be obtained. On the other hand, when the content of Ca exceeds 0.005%, coarse oxides are generated, resulting in the deterioration of SSC resistance. Therefore, the content of Ca to be added is set to 0.0002 to 0.005%.

Mg is an element effective for enhancing the hot workability of steel. However, when the content of Mg is less than 0.0002%, the said effect cannot be obtained. On the other hand, when the content of Mg exceeds 0.005%, coarse oxides are generated, resulting in the deterioration of SSC resistance. Therefore, the content of Mg to be added is set to 0.0002 to 0.005%.

La is an element effective for enhancing the hot workability of steel. However, when the content of La is less than 0.0002%, the said effect cannot be obtained. On the other hand, when the content of La exceeds 0.005%, coarse oxides are generated, resulting in the deterioration of SSC resistance. Therefore, the content of La to be added is set to 0.0002 to 0.005%.

Ce is also an element effective for enhancing the hot workability of steel. However, when the content of Ce is less than 0.0002%, the said effect cannot be obtained. On the other hand, when the content of Ce exceeds 0.005%, coarse oxides are generated, resulting in the deterioration of SSC resistance. Therefore, the content of Ce to be added is set to 0.0002 to 0.005%.

The above-mentioned Ca, Mg, La and Ce can be added singly or in a combination of two or more elements. Elements to be particularly preferably added among the above-mentioned elements are Ca and La.

From the above-mentioned reasons, the martensitic stainless steel for oil wells according to the said Invention (2) is regulated to contain, in lieu of part of Fe of the martensitic stainless steel for oil wells in the said Invention (1), either one or both elements selected from Nb: 0.020 to 0.045% and V:0.01 to 0.2%.

And the martensitic stainless steel for oil wells according to the said Invention (3) is regulated to contain, in lieu of part of Fe of the martensitic stainless steel for oil wells in the said Invention (1), one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005%.

Further the martensitic stainless steel for oil wells according to the said Invention (4) is regulated to contain, in lieu of part of Fe of the martensitic stainless steel for oil wells in the said Invention (1), either one or both elements selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2%, and one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005%.

(B) Heat Treatment:

A martensitic stainless steel pipe for oil wells sufficiently usable even in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas can be relatively easily manufactured, for example, by the said Invention (5) comprising “heating a steel pipe using a martensitic stainless steel having the chemical composition described in above (A) as a raw material to a temperature range of 920 to 1050° C. followed by air cooling, and performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point”.

(B-1) Heating Temperature before Air Cooling:

The heating temperature before air cooling is preferably set to 920 to 1050° C. When the said heating temperature is lower than 920° C., undissolved carbides may exist in the steel, and they may increase the dispersion of strength. On the other hand, at a temperature higher than 1050° C., the microstructure may be coarsened, resulting in a deterioration in low temperature toughness. Therefore, in the said Invention (5), the heating temperature before air cooling is set to 920 to 1050° C.

(B-2) Tempering Temperature:

After heating to the temperature range of 920 to 1050° C. described in above (B-1) followed by air cooling, tempering is preferably performed at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point. Tempering at high temperature removes the internal stress of the martensite generated by air cooling, leading to improvement in the steel product properties. Particularly, satisfactory steel product properties can be obtained by tempering at not lower than 625° C. However, if the tempering temperature exceeds the Ac₁ point, the strength may be seriously changed. Therefore, the tempering temperature after air cooling is set to not lower than 625° C. and not higher than the Ac₁ point in the said Invention (5).

PREFERRED EMBODIMENT

The present invention will be described in more detail in reference to preferred embodiment.

EXAMPLE

Steels 1 to 29 having chemical compositions shown in Table 1 were melted and then cast by the continuous casting process. Steels 1 to 23 in Table 1 are steels of inventive examples having chemical compositions within the ranges regulated by the present invention. On the other hand, steels 24 to 29 are steels of comparative examples having chemical compositions out of the conditions regulated by the present invention.

[Table 1] TABLE 1 Steel Chemical compositions (mass %) Balance: Fe and impurities No. C Si Mn P S Cu Cr Ni Mo Ti Al  1 0.20 0.23 0.48 0.011 0.0010 0.01 12.54 0.11 0.01 0.002 0.001  2 0.20 0.23 0.49 0.013 0.0015 0.06 12.57 0.10 0.01 0.003 0.003  3 0.20 0.23 0.48 0.014 0.0009 0.01 12.54 0.12 0.01 0.002 0.008  4 0.20 0.21 0.49 0.013 0.0008 0.08 12.51 0.09 0.01 0.003 0.001  5 0.19 0.25 0.48 0.014 0.0014 — 12.58 0.11 0.01 0.002 0.001  6 0.20 0.23 0.49 0.013 0.0013 0.01 13.20 0.19 0.01 0.004 0.002  7 0.20 0.24 0.47 0.012 0.0010 0.01 12.60 0.13 0.01 0.002 0.001  8 0.18 0.21 0.90 0.013 0.0056 0.01 12.53 0.08 0.17 0.002 0.001  9 0.18 0.24 0.87 0.015 0.0051 0.01 12.47 0.14 0.15 0.003 0.001 10 0.18 0.21 0.90 0.017 0.0049 0.01 12.51 0.08 0.16 0.002 0.009 11 0.19 0.26 0.48 0.013 0.0008 0.01 12.43 0.17 0.01 0.001 0.009 12 0.19 0.23 0.56 0.011 0.0008 0.01 12.44 0.16 0.01 0.001 0.008 13 0.21 0.22 0.46 0.013 0.0010 — 12.48 0.11 0.01 0.003 0.001 14 0.20 0.23 0.47 0.013 0.0010 0.01 12.54 0.10 0.01 0.002 0.001 15 0.20 0.23 0.46 0.013 0.0010 0.01 12.54 0.10 0.01 0.002 0.009 16 0.20 0.23 0.44 0.017 0.0010 0.01 12.54 0.10 0.01 0.002 0.004 17 0.20 0.22 0.48 0.013 0.0010 — 12.50 0.11 0.01 0.003 0.001 18 0.19 0.22 0.49 0.014 0.0009 0.01 12.60 0.09 0.01 0.002 0.001 19 0.20 0.22 0.49 0.014 0.0005 0.01 12.57 0.10 0.01 0.001 0.001 20 0.20 0.25 0.48 0.013 0.0005 0.01 12.56 0.10 0.01 0.002 0.002 21 0.19 0.25 0.55 0.013 0.0005 0.02 12.56 0.08 — — 0.009 22 0.19 0.22 0.48 0.012 0.0009 0.01 12.53 — 0.01 0.001 0.001 23 0.20 0.23 0.47 0.011 0.0010 — 12.55 0.12 0.01 0.001 0.002 24 0.19 0.25 0.56 0.013 0.0008 — 12.43 0.17 0.01 0.001 *0.032 25 0.19 0.25 0.56 0.014 0.0011 — 12.40 0.17 0.01 0.001 *0.067 26 0.19 0.25 0.56 0.013 0.0008 0.01 12.43 0.17 0.01 0.001 *0.065 27 0.19 0.25 0.54 0.018 0.0008 0.01 12.43 0.17 0.01 0.001 *0.074 28 0.19 0.25 0.56 0.013 0.0008 0.01 12.43 0.17 0.01 0.001 *0.053 29 0.19 0.25 0.57 0.015 0.0008 0.01 12.43 0.17 0.01 0.001 *0.069 Steel Chemical compositions (mass %) Balance: Fe and impurities Ac

point No. N V Nb Ca Mg La Co (degrees C.)  1 0.0320 — — — — — — 795  2 0.0303 — — — — — — 795  3 0.0315 — — — — — — 795  4 0.0320 — — — — — — 795  5 0.0281 — 0.002 0.0010 — — — 795  6 0.0294 — — — — — — 805  7 0.0300 — — — — — — 795  8 0.0411 0.17 0.001 — — — — 785  9 0.0420 0.17 0.001 — — — — 785 10 0.0405 0.16 0.001 — — — — 785 11 0.0487 0.03 — — — — — 790 12 0.0485 0.03 — — — — — 790 13 0.0287 — — 0.0014 — — — 800 14 0.0297 0.03 0.002 0.0005 — — — 795 15 0.0303 0.03 0.002 0.0005 — — — 795 16 0.0301 0.03 0.002 0.0005 — — — 795 17 0.0287 0.04 0.030 0.0010 — — — 795 18 0.0321 0.03 0.001 — 0.0010 — — 790 19 0.0299 0.03 0.002 — — 0.0010 — 790 20 0.0311 0.04 0.002 — — — 0.0040 790 21 0.0287 0.07 0.002 0.0010 — — — 790 22 0.0312 — — — — — — 800 23 0.0295 — — — — — — 795 24 0.0487 — — — — — — 790 25 0.0487 — — — — — — 790 26 0.0477 0.03 — — — — — 790 27 0.0471 0.03 — — — — — 790 28 0.0487 0.03 — 0.0008 — — — 790 29 0.0481 0.03 — 0.0008 — — — 790 Note: The mark * shows out of range defined in the present invention.

A bloom of each steel was subjected to hot forging and hot rolling to produce a steel pipe with an outer diameter of 130 mm and a wall thickness of 8.24 mm.

The thus-obtained steel pipe was heated to a temperature shown in Table 2 followed by air cooling and tempering, and various test pieces were cut out therefrom in order to examine tensile properties, toughness and SSC resistance.

1. Tensile Properties:

Round bar tensile test pieces, each having a diameter of 6.35 mm and a parallel part length of 25.4 mm, were sampled in parallel in a longitudinal direction from the wall thickness center part of each steel pipe, and subjected to a tensile test at room temperature to measure the yield strength (YS).

2. Toughness:

V-notch test pieces, each having a width of 10 mm, regulated in JIS Z 2202 (1998) were sampled in parallel in a longitudinal direction from the wall thickness center part of each steel pipe, and subjected to a Charpy impact test at −40° C. to determine an absorbed energy (vE-₄₀).

3. SSC resistance:

C-method test pieces regulated in TM 0177-96 of NACE were sampled from each steel pipe, and a C method test of NACE was carried out in an environment in which mixed gas having a partial pressure of hydrogen sulfide of 30397.5 Pa (0.3 atm) and a partial pressure of nitrogen gas of 70927.5Pa (0.7 atm) is blown into a 5% NaCl aqueous solution of 30° C. As the test stress, 100% of the YS previously measured was taken. The SSC resistance was evaluated depending on whether the test piece is cracked or not in testing in a condition having a test time of 720 hours.

The results of the above-mentioned tests are collectively shown in Table 2. In the column of “SSC resistance”, a test piece free from cracking is shown by “◯”, and a one causing cracking by “x”.

[Table 2] TABLE 2 Heating temperature Tensile Tough- Before Air Tempering properties ness SSC Test Steel Cooling Treatment [YS] [vE-₄₀] resist- No. No. (degrees C.) (degrees C.) (MPa) (J) ence 1 1 950 695 571 91 ◯ 2 2 950 695 573 93 ◯ 3 3 950 695 588 89 ◯ 4 4 950 695 585 92 ◯ 5 5 950 695 575 94 ◯ 6 6 950 695 574 120 ◯ 7 7 950 695 579 101 ◯ 8 8 950 740 628 88 ◯ 9 9 950 740 648 75 ◯ 10 10 950 740 612 80 ◯ 11 11 950 740 618 80 ◯ 12 12 950 740 614 75 ◯ 13 13 950 705 589 88 ◯ 14 14 950 705 591 85 ◯ 15 15 950 705 590 90 ◯ 16 16 950 705 592 90 ◯ 17 17 950 705 602 98 ◯ 18 18 950 705 586 94 ◯ 19 19 950 705 602 93 ◯ 20 20 950 705 603 90 ◯ 21 21 950 740 605 72 ◯ 22 22 950 695 565 70 ◯ 23 23 950 695 570 90 ◯ 24 1 1010 705 585 85 ◯ 25 8 1010 745 633 81 ◯ 26 *24 960 700 618 34 X 27 *25 960 700 615 18 X 28 *26 960 700 621 15 X 29 *27 960 700 624 18 X 30 *28 960 700 631 21 X 81 *29 960 700 617 11 X 32 1 #1060 700 588 52 ◯ 33 1 #895 650 560 50 ◯ 34 1 960 #600 630 56 ◯ 35 8 #1060 760 610 50 ◯ Note: The mark * shows a steel out of range defined in the present invention in terms of its chemical composition. The mark # shows out of range defined in the invention (5).

It is apparent from Table 2 that Test Nos. 1 to 25 and Test Nos. 32 to 35, the chemical compositions of which satisfy the condition regulated by the present invention, have satisfactory strength-toughness balance, and have also excellent SSC resistance, causing no cracking in the execution of C-method test of NACE in the above condition. Among them, particularly, Test Nos. 1 to 25, which satisfy the manufacturing condition regulated by the said Invention (5), have satisfactory strength-toughness balance and SSC resistance, and further satisfactory toughness of vE-₄₀≧70J.

On the contrary, Test Nos. 26 to 31, the chemical compositions of which are out of the condition regulated by the present invention, cause cracking in the execution of C-method test of NACE in the above condition, and are apparently inferior in SSC resistance.

Although only some exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciated that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present invention. Accordingly, all such modifications are intended to be included within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The martensitic stainless steel for oil wells of the present invention can be used in an environment containing hydrogen sulfide, carbon dioxide and chloride ion in cold areas since it is excellent in low temperature toughness as well as satisfactory SSC resistance in the coexistence of chloride ion, wet carbon dioxide and trace hydrogen sulfide. According to the method for manufacturing a martensitic stainless steel pipe for oil wells of the present invention, a steel pipe which stands good for a use in the above-mentioned severe environment can be easily manufactured. 

1. A martensitic stainless steel for oil wells, which comprises by mass percent, C: 0.16 to 0.22%, Si: 0.1 to 0.8%, Mn: 0.25 to 1.00%, P: not more than 0.025%, S: not more than 0.010%, Cr: 12.0 to 13.5%, Al: not more than 0.010%, Ni: 0 to 0.2%, Cu: 0 to 0.10%, Mo: 0 to 0.20%, Ti: 0 to 0.050% and N: 0.01 to 0.1%, with the balance being Fe and impurities.
 2. The martensitic stainless steel for oil wells according to claim 1, which further contains either one or more elements selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2% in lieu of part of Fe.
 3. The martensitic stainless steel for oil wells according to claim 1, which further contains one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005% in lieu of part of Fe.
 4. The martensitic stainless steel for oil wells according to claim 1, which further contains either one or both elements selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2%, and one or more elements selected from among Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: 0.0002 to 0.005% in lieu of part of Fe.
 5. A method for manufacturing a martensitic stainless steel pipe for oil wells, which comprises: heating the martensitic stainless steel pipe, having the chemical composition according to claim 1, to a temperature range of 920 to 1050° C. followed by air cooling; and then, performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point.
 6. A method for manufacturing a martensitic stainless steel pipe for oil wells, which comprises: heating the martensitic stainless steel pipe, having the chemical composition according to claim 2, to a temperature range of 920 to 1050° C. followed by air cooling; and then, performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point.
 7. A method for manufacturing a martensitic stainless steel pipe for oil wells, which comprises: heating the martensitic stainless steel pipe, having the chemical composition according to claim 3, to a temperature range of 920 to 1050° C. followed by air cooling; and then, performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point.
 8. A method for manufacturing a martensitic stainless steel pipe for oil wells, which comprises: heating the martensitic stainless steel pipe, having the chemical composition according to claim 4, to a temperature range of 920 to 1050° C. followed by air cooling; and then, performing tempering treatment at a temperature ranging from not lower than 625° C. to not higher than the Ac₁ point. 